The invention relates to a method and arrangement for detecting payload data encoded into relative positions and/or relative polarities of watermarks embedded in an information signal.
International Patent Application WO-A-99/45705 discloses a method of detecting multi-bit payload data in an information signal. The payload is encoded into the relative positions and polarities of multiple (possibly equal) embedded watermark patterns. The prior-art detection method comprises computing the correlation of the embedded watermark patterns and the information signal as a function of the position of the watermark patterns with respect to the signal. This computing step yields a correlation function in which the positions and polarities of the embedded watermark patterns are represented by large positive or negative peaks. The relative distance between, and the signs of, the peaks represent the payload data.
In the ideal case, the peaks of the correlation function resemble Dirac-pulses and their positions can be determined with sufficient precision. However, in many practical situations the positions and the polarities are not so well defined. This is particularly the case if the signal has been subjected to processing. Video processing, for example, causes the correlation peaks to be smeared. Peaks are broadened, so that the central position is no longer well defined. Even the sign of a peak may be incorrectly retrieved due to large negative side lobs.
As a solution to this problem, WO-A-99/45705 teaches restriction of allowed peak positions to a sub-grid of all possible positions. Peak jitter is then resolved by quantizing to the nearest sub-grid point. However, the payload capacity of the watermark is thereby reduced. Moreover, it does not solve the inversion of peak polarity.
It is an object of the invention to provide an improved method and arrangement for detecting the distance between multiple watermark patterns embedded in an information signal. It is also an object of the invention to provide an improved method and arrangement for detecting the relative polarities of said watermark patterns.
To this end, the step of determining the relative positions of peaks comprises computing the resemblance of the correlation function in a window around one of said peaks to the correlation function in a window around another one of said peaks for a plurality of positions of at least one of said windows, and determining the positions of said windows for which said resemblance has an extreme value.
The invention is based on the recognition that if peaks of a correlation function are smeared, they are all smeared in the same manner. The correlation functions (the waveforms) in the neighborhoods of peaks resemble each other to a large extent. The invention exploits this property by identifying the neighborhoods in which the correlation functions resemble each other most, and determining the distance between the identified neighborhoods rather than the distance between individual peaks.
The same solution applies to the detection of the relative polarities of the peaks. In accordance with the invention, the step of determining the relative polarities of peaks comprises computing the resemblance of the correlation function in a window around one of said peaks to the correlation function in a window around another one of said peaks, and determining the sign of said resemblance value. If the sign is positive, the two peaks have the same polarity. If the sign is negative, the peaks have opposite polarities.